In-mold coating is a known technique for decorating or priming the surface of a molded article formed in a matched die mold under heat and pressure. The coating powder, which can be an unsaturated polyester formulation, is applied as a powder to the interior wall of the mold, preferably electrostatically. The powder melts due to the heat of the mold and coalesces on the mold surface to produce a uniform coating. The fill resin which constitutes the bulk of the finished article is then inserted in the mold and molded. When the cures of the coating and fill resin have advanced sufficiently, and the coating and fill resins are integral, the mold is opened and the molding removed. It has been found desirable that the coating and fill resins be crosslinked across the interface between them to maximize adhesion.
In-mold coating as described above is distinguished from post-mold coating processes, in which the fill resin is molded before the coating is introduced into the mold, and conventional decorating operations in which the fill resin is molded and cured in the mold, then released from the mold and decorated with a powder coating or other finish. Also to be distinguished are the gelcoat or liquid in-mold coating processes, in which a liquid resin is applied to the mold surface prior to molding a part. The gelcoat or liquid in-mold coating processes are not widely used for matched die molding of unsaturated polyester articles because the coating is hard to apply uniformly to the mold surface, overspray must be collected and removed, and the solvents or liquid monomers of the coating tend to flash away when sprayed on the hot mold, thus changing the coating formulation, creating bubbles and other irregularities in the coating, and making it necessary to contain and collect the vapor of volatile ingredients.
The following patents relate to in-mold coating processes of the type taught herein:
______________________________________ U.S. Pat. No. First Inventor Issued ______________________________________ 3,216,877 Mohr 11/9/65 4,205,028 Brueggemann 5/27/80 4,228,113 Van Gasse 10/24/80 4,287,310 Van Gasse 9/1/81 4,315,884 Van Gasse 2/16/82 4,316,869 Van Gasse 2/23/82 4,349,601 Brueggemann 9/14/82 4,499,235 Verwer 2/12/85 ______________________________________
British specification No. 1,420,867, published Jan. 14, 1976, teaches an analogous process except that the fill resin is first formed and apparently cured, then it is removed from the mold, the mold is powder coated, and the article is returned to the mold and remolded to apply the coating. Netherlands patent No. 7903-428, published Apr. 11, 1980, is believed to teach a similar process.
Of all the known references to initiators useful for curing polyester compositions, only U.S. Pat. No. 4,331,735 (issued May 25, 1982 to Shanoski) and the LUCIDOL General Catalog--Peroxides and Specialty Chemicals dated January, 1984 indicate the utility of 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane for curing polyester compositions generally. (LUCIDOL is a trademark of Pennwalt Corporation, Buffalo, N.Y.) Neither of these references recites a particular composition containing this initiator or indicates any preference for this initiator.
Several patents suggest using more than one peroxide initiator in a single resin composition, but none of them pertains to powder coating resins, and particularly to unsaturated polyester powder coating resins. The relevant patents of this type are as follows:
______________________________________ U.S. Pat. No. First Inventor Issued ______________________________________ 4,123,488 Lawson 10/31/78 4,331,735 Shanoski 5/25/82 4,515,710 Cobbledick 5/7/85 3,230,284 Iverson 1/18/66 ______________________________________
The Shanoski patent, column 5, lines 56-59, and Cobbledick at column 5, lines 31-34, each state, "It is desirable sometimes to use mixtures of initiators to take advantage of their different decomposition rates and times at different temperatures and so forth." No criteria for selecting a mixture of initiators are given. Example IV of Shanoski employs a combination of 1.87 parts by weight of t-butylperbenzoate (our species AJ in Table I below) and 0.206 parts by weight t-butylperoctoate (our species W in Table I below) per 100 parts by weight of resin as an initiator for a liquid in-mold coating. The former initiator is a slow initiator (except for its small proportion) and the latter initiator is a fast initiator as defined below. The Cobbledick reference does not employ more than one initiator at a time in its working examples. Iverson teaches an unsaturated polyester based coating, initiated with a solution of 100 parts by weight of methylethylketone peroxide (60% in dimethylphthalate) and 3 parts by weight of peroxyacetic acid. Peroxyacetic acid explodes at 110.degree. C., according to the Condensed Chemical Dictionary, 10th Ed.
Referring once again to the prior art relating to single initiator in-mold coating powders, the prior technology ignores the different effects of fast and slow initiators as defined later herein, and does not reveal that the use of a fast initiator alone does not provide an acceptable coating, while the use of a slow initiator alone at common molding temperatures requires a large proportion of the initiator or lengthens the mold cycle time undesirably by slowing the cure of the coating. The art has also not appreciated the absence of acceptable moderate rate initiators available for use as single initiators in coating powder compositions for in-mold coating. Another disadvantage of many prior art initiators is their short shelf life, either alone or when formulated into coating powders. Many of these initiators and their resulting compositions must be refrigerated prior to use, and in many cases the initiator must be mixed with the balance of the composition shortly before use.